The disclosure relates generally to gain regulation of a nuclear radiation detector, and more particularly to methods and apparatus to regulate the gain based on the gamma ray background generated in the apparatus by inelastic collisions of fast neutrons with the nuclei of some of the elements constituting the apparatus material.
Detectors often play a role in many nuclear measurements. This can include, among others, neutron measurements in industrial applications, homeland security, neutron physics and also in oil well logging measurements using neutron sources. At present, two kinds of detectors may be used in downhole tools. One type of detector can serve to detect fast neutrons and may employ a plastic scintillation detector. A second, more common type can serve to detect thermal or epithermal neutrons such as a 3He detector or, less frequently, a scintillation detector using 6Li-glass. 3He detectors can be virtually insensitive to gamma-rays. 6Li detectors on the other hand may have significant gamma-ray sensitivity and suppression or subtraction of gamma-ray induced background in the presence of gamma-rays from inelastic neutron interactions or neutron capture can be difficult and prone to error.
These types of detectors are used in a multitude of downhole tools. The basic application is in the measurement of neutron porosity through the detection of thermal or epithermal neutrons. Other applications may include the determination of neutron-gamma-density (see U.S. Pat. No. 5,608,215 and U.S. Pat. No. 5,804,820, assigned to the assignee of the present disclosure). In addition, the present scarceness of 3He, a gas that is widely used in thermal and epithermal neutron detectors, has made alternatives for neutron detection to be of practical interest.
Turning now to background for the present disclosure, gamma ray detectors are used for many nuclear measurements. The usable information from these detectors falls in one or more of the following categories: the number of detected gamma rays, the energy of the detected gamma rays, and the arrival time of detected gamma rays. Equipment employing gamma ray detectors should deliver same or similar answers independent of environmental conditions (e.g. temperature) and changes in the performance of individual detectors and their components (e.g. PMT gain as a function of applied high voltage and scintillator light output).
One operational parameter that may be adjusted for gamma ray detectors is the Photomultiplier Tube (PMT) high voltage. Adjusting the PMT voltage changes the multiplication (or gain) of the electrons that are created when light from the scintillator hits the photocathode. The high voltage can be adjusted to correct for temperature effects, such as a change in the efficiency of the photocathode and/or a change in the scintillation properties of the detector crystal. The high voltage setting on the PMT does not notably affect the detection of the arrival time of gamma rays. The number and energy of detected gamma rays are both sensitive to the PMT high voltage.
A gamma ray is detected upon interaction with the scintillation crystal in the detector, creating light photons. These photons may liberate electrons from the photocathode, which may in turn be accelerated and multiplied in the photomultiplier. The current from the anode of the PMT may be converted to a voltage, which may be digitized and used as an indication of the energy deposited by the gamma ray in the scintillator.
To suppress noise, there can be defined an electronic threshold, below which the digitized signal is discarded and not counted as a gamma ray. If the high voltage setting on the PMT is incorrect relative to this threshold, more or fewer gamma rays may be counted than should be and a response based on counting the total number of gamma-rays may be altered.
An accurate and precise energy calibration of gamma ray detectors may be used for nuclear measurements performed, for example, during well logging. Traditionally, energy calibration, or gain regulation, may be performed by analyzing the position of a reference gamma ray line in the detector energy spectrum. A 137Cs radioactive source may be added for the purpose of generating the reference gamma ray line. The 137Cs radioactive calibration source delivers a narrow gamma ray line at 662 keV. The energy of this gamma ray line, however, as well as the typical activity of a micro-stabilization source (some kBq), may not be adequate for some types of measurements. Additionally, there is an ongoing effort to limit the use of radioisotopic sources in nuclear tools for reasons of personnel safety and national security.